PROJECT SUMMARY Children with chronic medical conditions are at risk of compromised bone strength and increased fracture susceptibility. Recently developed reference data have enabled clinicians to identify children 5 to 20 y with compromised bone mineral status (low bone density for age) and optimize their medical care. A glaring deficit is the inability to evaluate bone health of young children < 5 y due to lack of appropriate bone density reference data, although numerous clinical conditions (e.g., prematurity, gastrointestinal and neuromuscular disorders) threaten bone health in this young age range. Early childhood differs from other developmental stages. In older children, the need to account for growth status and lean mass when interpreting bone measurements has been demonstrated, but in early childhood, these effects are unknown. Pronounced changes in gross motor skills, locomotion and physical activity also occur in early childhood. Mechanical loads on the growing skeleton promote bone mineral accrual. Understanding the impact of growth, body composition, motor skill development, and physical activity on bone accrual in young children is critical for interpretation of a clinical bone mineral status measurement. These characteristics are often affected in young children with chronic medical conditions. The long-term goal of this study is to optimize bone health of young children. The objectives of this longitudinal study are to develop regional and total body bone mineral content (BMC) and density (aBMD) reference data for children ages 1-5 y to aid identification of young children with bone deficits, and identify factors that influence bone accrual in this age group. The aims of this longitudinal study are: (1) test for sex and ancestry differences in BMC and aBMD at multiple skeletal sites in healthy children ages 1 to 5 y, and create appropriate reference curves; (2) assess the influence of growth and body composition, and (3) gross motor skills and physical activity on bone mineral status and bone mineral accrual, and (4) determine the degree of tracking of bone status in young children. Hypotheses to be tested are: males will have greater BMC and aBMD than females; Blacks will have greater BMC and aBMD than non-blacks; bone mineral status and bone accrual will be greater in children with greater growth status and more lean tissue, more advanced gross motor skills, and higher physical activity levels; and BMC and aBMD z-scores will be positively correlated over 3 y. The study is unique since no previous studies have addressed these issues in children 1-5 y. It is clinically relevant as findings will facilitate medical care of children with chronic medical conditions that threaten bone health, and it will develop and validate novel methods for bone and body composition assessment in this age range. It will also add generalizable knowledge to enhance understanding of the growing skeleton, and its relationship to motor and somatic development. Since poor bone mineral accrual can have lifelong consequences, this study addresses the major public health concern of osteoporosis.